Compared to other magnetic materials, NdFeB magnetic materials have excellent magnetic and mechanical properties. Therefore, they have been applied in many fields, such as electronic information, household appliances, medical treatment, aerospace, and especially in the new green energy fields of energy conservation vehicles and wind power. These wide application fields also bring the rapid increase in annual output of NdFeB magnets. Consequently, the NdFeB wastes, including the scraps and sludge that are generated during the manufacture processes, are about 30 wt. % of the as-sintered materials. China, for example, as the largest manufacturer of NdFeB magnets, had an annual output of about 94 thousand tons in 2013, which accounted for 91% of the global output. At the same time, about 20-30 thousand tons of NdFeB raw materials were formed into sludge during the production process. With the development of global environmental legislation, as well as the requirement of resources protection and sustainable development, the recycling of waste NdFeB materials has become very important. As the price of rare-earth metals and the fabrication costs have increased over the years, the green and efficient recycling of waste NdFeB materials could not only protect the environment and save resources, but also bring substantial economic and social benefits.
At the present time, possible routes to recycle scraps of sintered NdFeB magnets are: (1) Hydrogen decrepitation to get powders, followed by coating with rare earth rich powders, alignment, and bonding or hot pressing into bulk magnets; (2) Hydrogenation, disproportionation, desorption, recombination (HDDR) process to obtain high coercivity powders for bonding or hot pressing; (3) Milling, alignment, and vacuum sintering into bulk magnets; (4) The powders could be blended with other fresh powders and processed by one of the ways above, but the magnetic properties would drop accordingly.
On the other hand, the recycle technology of NdFeB rare earth permanent magnet sludge waste is currently a hydrometallurgical process. These processes are comprised of the following: acid dissolution-precipitation process, complex salt conversion process, hydrochloric acid dissolved superior process, and full extraction processes. Various methods are briefly compared as follows: (1) Acid dissolution-precipitation process: This process belongs to relatively primitive methods. Main procedures include oxidizing roasting, acid decomposition, precipitation, burning to achieve rare earth oxides, subsequently electrolyzing rare earth fluoride to prepare pure metal. The recovery rate of rare earth oxides in batch production is low. (2) Hydrochloric acid dissolved superior process: This process is divided into oxide roasting, decomposition and purification, extraction and separation, and sedimentation burning. The recovery rate of rare earth is more than 95%, the purity of Dy2O3 is 99%, and the purity of Pr2O3 is 98% by this method. Furthermore, the raffinate can achieve precipitate of rare earth carbonate polymorphs which can meet customers' demands. (3) Sulfuric acid complex salt precipitation process: This process typically includes the following steps: sulfuric acid dissolution, complex salt precipitation of rare earths, alkali conversion, hydrochloric acid dissolution, extraction and separation, precipitation, and burning to obtain rare earth oxides. Complex salt conversion process could separate Nd2O3 and non-rare earth (Fe, Al, etc.). By this method, the purity of rare earth oxides could reach 93%. The recovery rate of Nd2O3 in final product is high (up to 85.53%), and the purity of Nd2O3 and Dy2O3 is 99%. Therefore, this process is widely used in the industry nowadays. (4) Full extraction process. Full solvent extraction processes of NdFeB waste are: extraction of iron by N503, extraction of rare earth by P507, separation of neodymium and dysprosium, further purification of cobalt. After 60 levels segment extraction test, Nd2O3 with 99% purity, Dy2O3 with 98% purity, and cobalt carbonate product with 99% purity are achieved. However, this process needs more steps and a longer production cycle. The final products of above process are rare earth oxide or metal, and the above-mentioned processes have common disadvantages of long flow, generation of a large amount of waste acid that pollutes the environment.
To solve these problems, China patent (Application No. 201410101544.7) disclosed a method for preparing the recycled NdFeB magnetic powders form NdFeB sludge. By this method, NdFeB powders could be obtained from the NdFeB sludge, but the resultant magnetic powders did not have the desired magnetic properties and cannot be directly used in applications.